1. Field of the Invention
The present invention relates to an optical information recording/reproducing apparatus for recording information or the like by using an optical information recording medium, and, more particularly, to an optical information recording/reproducing apparatus for performing a tracking servo and/or a focus servo.
2. Description of the Related Art
The structures of an optical information recording/reproducing apparatuses will be described. A first related art arranged in such a manner that a diffraction grating is employed as a luminous separation means of a light is constituted in such a manner that laser beams emitted from a semiconductor laser are paralleled by a collimate lens, then the laser beams are made incident upon a beam splitter. In this case, since the direction of the polarization of the incidental a light is parallel to the grooves formed in the beam splitter, the light is diffracted before the light is made incident upon a quarter wavelength plate, then the light becomes a circularly polarized wave. The circularly polarized wave is converged by a lens so as to be irradiated to an optical disk. As a result, recording or the like of information is performed. A light reflected from the optical disk is converted into a linearly polarized wave by the quarter wavelength plate, then the wave transmits the beam splitter so as to be introduced into a signal detecting optical system. Then the linearly polarized wave is made incident upon a critical angle diffracting grating. The incidental light twice generates the critical angle diffraction and total reflection to thereby become a diffracted light, then the light is made incident upon a light receiving element which is divided into four sections. Since the light is detected by the light receiving device, information is reproduced and a focus error signal and a track error signal are detected.
A second related art employs a dual type diffracting grating. A light emitted from a semiconductor laser are paralleled by a collimate lens, then are reflected by a beam splitter via two shaping prisms so as to be converged by an objective lens and are irradiated to a magneto-optical disk. As a result, recording of information or the like is performed. Light reflected by the disk is introduced into a signal detection optical system after the light transmits through the beam splitter so as to be converged by a convergent lens. Then, the light transmits a diffraction grating formed on the both sides of the diffraction grating or the light is diffracted. A 0-order light which is moved straight after the light transmits through the diffraction grating is introduced into a four-sectioned light receiving element. The diffracted 1-order light is introduced into a two-sectioned light receiving element. A magneto-optic signal which serves as a reproduction signal is detected by the difference in the quantity between the 0-order light and the 1-order light. A focus error signal is detected by using the 0-order light by an astigmatic method, while a track error signal is detected by using the 1-order light by a push-pull method or the like.
A third related art will be described. Beams emitted from a semiconductor laser, as a laser light source, are paralleled by a collimate lens. Then, the thus formed parallel light is reflected by an optical path separation means so as to be converged by an objective lens, then is irradiated to a magneto-optical disk, as an optical information recording medium. As a result, recording of information is performed. A light reflected by the magneto-optical disk passes through the luminous separating means so as to be introduced into a signal detecting optical system, then is detected by a light receiving device. As a result, a reproduction of information and detections of a focus error signal and a track error signal are performed.
A fourth related art will be described.
A light emitted from a semiconductor laser is reflected by a diffraction grating formed on one side of a luminous separating element so as to be paralleled by a collimate lens. The thus formed parallel light is converged by an objective lens, then is irradiated applied to the surface of the magneto-optical disk. As a result, recording of information is performed. A light reflected by the disk successively transmits the objective lens and the collimate lens. Then, the reflected light passes through the diffraction grating of the luminous separating element so as to be divided into a 0-order light and an 1-order light. The thus divided 0-order light and the 1-order light are reflected by a total reflection surface formed on the reverse side of the luminous separating element, again transmit through the diffraction grating so as to be emitted outwards. As a result, the 0-order light and the 1-order light are caused to have different length of optical paths. Therefore, these lights are introduced respectively into two light receiving devices disposed on the same plane, so that each of the signals can be detected.
The problems experienced with a conventional optical information recording/reproducing apparatus will be described.
A first problem will be described. In the case of the first related art in which an additional writing type optical disk or a CD can be applied to an optical information recording medium, it has no function of detecting a magneto-optic signal. Therefore, it cannot be used as a rewriting optical disk head.
A second problem will be described. In the case of the second related art in which a dual type diffraction grating is employed so as to emit a 0-order light and an 1-order light, which pass through the dual type diffraction grating to thereby be formed, in substantially the same direction, the size of the signal detecting optical system can be, to a certain degree, reduced. However, the necessity of providing optical parts such as a beam splitter and a condenser lens cannot be eliminated. Therefore, the overall size and weight of the apparatus cannot be satisfactorily reduced.
A third problem will be described. In the case of the third related art in which a luminous flux separating means is employed so that a luminous flux including an emitted light emitted from a laser light source and a reflected light reflected from the magneto-optical disk are separated from each other, the luminous flux separating means usually comprises a prism such as a beam splitter. However, a subject to reduce the size and weight of the optical pickup optical system has become important recently for the purpose of shortening the access time. In order to solve this subject, it might therefore be feasible to employ a structure of the luminous flux separating means in which as dual diffraction grating having diffraction gratings on both sides thereof is employed. As a result of the employment of the dual diffraction grating, the number of the parts and the space for the overall body of the optical pickup optical system can be reduced. Furthermore, the seeking transfer can be speedy.
However, the diffraction grating formed on the dual type diffraction grating can be easily influenced by the change in the wavelength. What is even worse, the shape of the grating can be easily got out since the surface on which the diffraction grating is formed is brittle. Therefore, the accuracy of the wave surface for reflecting, transmitting and diffracting can be deteriorated. Therefore, the converging spot may be undesirably enlarged, causing an accurate information recording/reproducing not to be performed.
A fourth problem will be described. In the case of the second related art arranged in such a manner that the overall optical system is divided into the light emitting optical system and a signal detecting optical system, the parts for the overall body increase in number, causing the overall cost to be raised excessively.
A fifth problem will be described. In the case of the fourth related art in which a semiconductor laser and a light receiving device are integrally formed and the change in the diffraction angle against the change in the wavelength, by using a piece of diffraction grating, can be prevented similarly to the structure in which two diffraction gratings are used. The separation of the light illuminating the magneto-optical disk and the light reflected by the magneto-optical disk is determined depending upon the thickness of the substrate. Therefore, the design freedom is restricted by the interval between the light emitting point and the light receiving point and the focusing sensitivity. Furthermore, the adjusting, the assembling and the manufacturing of the elements cannot be easily performed. Furthermore, the light utilization efficiency can be reduced by the reflection of light on the surface of the diffraction grating.
A sixth problem will be described. In the case of the fourth related art in which a beam separating element having a diffraction grating and a total reflecting surface is employed, the number of the parts for the optical system can be reduced and the space can thereby be saved. However, the two light receiving elements for detecting the transmitted light and the diffracted light are mounted on one substrate in such a manner that the two light receiving devices hold the semiconductor laser therebetween. Therefore, it is excessively difficult to adjust closely the position of the above-described two light receiving elements and the semiconductor laser. Furthermore, since the distance between the mounting substrate and the semiconductor laser and the distance between the mounting substrate and the light receiving elements are different from each other, a height adjusting device must be additionally provided. Since the light receiving elements are disposed on the two sides of the semiconductor laser. There are problems that the assembling and the adjusting of the light receiving elements cannot be easily completed and take a long time.